Not within Numbers.
Possible with an AppleScript, may be possible with Automator. The script could run through these steps:
Open the template
Read the value in the cell containing the invoice number,
Add one to the value.
Insert the changed value in the same cell.
Save the result as a Template (using the same name)
Say 'yes' to the alert that warns the template already exists and asks it you want to overwrite it.
Opens the Save dialogue with "Invoice "and the current number inserted, and stops.
You then enter the customer name and click Save.
Translating that to a script is outside my knowledge area, but there are others here who might find it a good starting point.